[NET_SCHED]: Fix prio/ingress classification logic error

Fix handling of empty or completely non-matching filter chains. In
that case -1 is returned and tcf_result is uninitialized, the
qdisc should fall back to default classification in that case.

Noticed by PJ Waskiewicz <peter.p.waskiewicz.jr@intel.com>.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>